Re: [PATCHv4 12/24] thp: PMD splitting without splitting compound page

[Aneesh Kumar K.V]

"Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx> writes:

> Current split_huge_page() combines two operations: splitting PMDs into
> tables of PTEs and splitting underlying compound page. This patch
> changes split_huge_pmd() implementation to split the given PMD without
> splitting other PMDs this page mapped with or underlying compound page.
>
> In order to do this we have to get rid of tail page refcounting, which
> uses _mapcount of tail pages. Tail page refcounting is needed to be able
> to split THP page at any point: we always know which of tail pages is
> pinned (i.e. by get_user_pages()) and can distribute page count
> correctly.
>
> We can avoid this by allowing split_huge_page() to fail if the compound
> page is pinned. This patch removes all infrastructure for tail page
> refcounting and make split_huge_page() to always return -EBUSY. All
> split_huge_page() users already know how to handle its fail. Proper
> implementation will be added later.
>
> Without tail page refcounting, implementation of split_huge_pmd() is
> pretty straight-forward.
>
> Memory cgroup is not yet ready for new refcouting. Let's disable it on
> Kconfig level.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
.....
.....

>  static inline int page_mapped(struct page *page)
>  {
> -	return atomic_read(&(page)->_mapcount) + compound_mapcount(page) >= 0;
> +	int i;
> +	if (likely(!PageCompound(page)))
> +		return atomic_read(&page->_mapcount) >= 0;
> +	if (compound_mapcount(page))
> +		return 1;
> +	for (i = 0; i < hpage_nr_pages(page); i++) {
> +		if (atomic_read(&page[i]._mapcount) >= 0)
> +			return 1;
> +	}

do we need to loop with head page here ? ie,

page = compound_page(page);

> +	return 0;
>  }

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